Nick Diaz can't handle losing, and he knows he will. He has only the slightest chance of winning, but more likely he will be humiliated for the entire fight.
Just look how he acts when he loses: Riggs, Noons, Condit, je just can't take it and he knows in the depths of his heart that GSP is just another loss on Nick's record. I really don't think he's mature enough to show up, but others label that immaturity as 'anxiety'.